The present invention relates to an improved control-screen electrode subassembly for an electrostatically-focused electron gun for a cathode-ray tube, CRT, and particularly to an integral control-screen electrode subassembly having improved stability and alignment between two adjacent electrodes.
An electron gun, such as used in a display CRT and a projection kinescope, is designed to generate and direct an electron beam to a small area on the screen of a tube. The electron gun has a cathode assembly and a series of apertured electrodes spaced therefrom. The electrodes are held in place relative to each other by separate attachment to a plurality of glass support rods. Such a structure is shown in U.S. Pat. No. 2,275,029 issued to Epstein on Mar. 3, 1942 and in U.S. Pat. No. 3,004,186 issued to Gray on Oct. 10, 1961. The electrode closest to the cathode assembly is called the G1 and is usually a control grid. The next electrode is called the G2 and is usually a screen grid. The spacing between these two grids is critical for maintaining the operating voltages within the established limits. Furthermore, the G1 and G2 electrode apertures must be precisely aligned in order to provide minimum aberrations from the beam forming region and to ensure alignment with the main lens axis.
In the above-referenced Epstein and Gray patents, the cathode is constructed as a subassembly with a cup-shaped control grid electrode. This subassembly is attached, in an operation called beading, to the glass support rods separately from the screen grid attachment. Since the beading operation is performed at an elevated support rod temperature, the G1 to G2 electrode spacing and aperture alignment may be altered during the subsequent cooling of the support rods. During tube operation, the glass support rods also become heated and expand. Since the support rods are separated from each other, the heating of the rods may be somewhat dissimilar thereby causing a difference in expansion resulting in a variation in electrode spacing and aperture alignment.
U.S. Pat. No. 2,825,837 issued to Dudley on Mar. 4, 1958 shows an electron gun in which the G1 electrode and the G2 electrode are slidably attached to a plurality of steatite support rods and spaced apart by steatite washers disposed between the support flanges of the electrodes. In the Dudley structure, the precision of the electron beam aperture alignment depends upon the accuracy with which the support flanges are attached to the G1 electrode and the tolerances of the steatite support rods as well as the tolerance of the support apertures in the G2 electrode and the G1 support flanges.
U.S. Pat. No. 2,170,663 issued to Painter on Aug. 22, 1939 shows an electron gun structure comprising a plurality of electrodes and insulating spacers compressively retained between flanged discs and welded to a cylindrical electrode. The Painter structure provides accuracy of alignment and of spacing of the various tube parts; however, the Painter structure is complex and expensive.
U.S. Pat. No. 2,375,815 issued to Ohl on May 15, 1945 shows a monolithic electron gun structure comprising a cathode assembly, control electrode and anode attached to an insulating member. The spacing between the control electrode and the anode as well as the aperture alignment is established by precisely forming a pair of steps in the insulating member. The spacing and alignment between the adjacent electrodes depends upon the accuracy with which the step height as well as the step width can be formed. It is well known that precisely-formed insulators, such as ceramics, are expensive. Precisely-formed annular ceramic rings for spacing the control electrode from the adjacent electrode are also used in U.S. Pat. No. 3,979,631 issued to van der Ven on Sept. 7, 1976 and in U.S. Reissue No. 25,127 issued to Szegho on Feb. 20, 1962. In the Szegho reissue patent, a pair of ceramic spacing rings are utilized. One ring, having an accurately-controlled outside diameter and thickness, establishes the G1 to G2 longitudinal spacing while a second ceramic ring, having precisely-formed, concentric inside and outside diameters, provides the lateral spacing and thus the aperture alignment between the electrodes. Such a structure, while apparently simple in construction, is very expensive and therefore impractical.
In U.S. Pat. application, Ser. No. 70,738 of McCandless, filed on Aug. 29, 1979 now U.S. Pat. No. 4,298,818 issued on Nov. 3, 1981, and assigned to the same assignee as the present invention, an improved electron gun is disclosed. The McCandless electron gun comprises a plurality of cathode assemblies attached to one surface of a single, flat ceramic wafer and two electrodes (G1, G2) individually attached to the opposite side of the ceramic wafer. In the McCandless structure, the control grid is brazed to the ceramic wafer along a peripheral rib while the screen grid is brazed to the ceramic wafer at the ends of two parallel flanges. Thus, the spacing between the control grid and the screen grid electrodes is directly related to the height of the peripheral rib and the length of the flanges since each contacts the same surface of the ceramic wafer. The McCandless structure thus transfers the cost and precision from the ceramic member to the two-tool formed metal parts.